Stallion Infertility and the Y Chromosome

Environmental factors, management, and genetics all play a part in determining a stallion's fertility. Bhanu P. Chowdhary, BVSc, AH, MVSc, PhD, associate professor in animal genomics at Texas A&M University's College of Veterinary Medicine, has turned his attention to the role of genetics, specifically on the Y chromosome, in finding causes of stallion infertility. He presented information on his current research on this topic, and his long-term goals to be able to genetically diagnose stallions with Y chromosome-related fertility problems, at a recent University of Kentucky Gluck Equine Research Center lecture for equine researchers, students, and practitioners.

According to Chowdhary, the Y chromosome was largely ignored, compared to other chromosomes, because it was thought to only determine sex. It was often called a "genetic wasteland, junkyard, a barren chromosome, an 'empty dance partner' for X, a genetic couch potato, an adolescent tearaway, and a degenerate," he said.

In June 2003, after scientists deciphered the role of genes on the human Y chromosome, Chowdhary said the chromosome was "suddenly considered to be a 'goldmine,' 'crystal palace,' and 'hall of mirrors,' " because of its bizarre organization and potential role in causing male infertility, sex reversal, gonadal dysgenesis (in which an infected individual looks like female but has male chromosome composition, specifically a defective Y chromosome), aging, and cancer. Y chromosome DNA can also be useful for forensic analysis and paternity testing. "Finally the Y chromosome got some stardom, and wasn't just referred to as the sex-determining chromosome," he said.

Six percent of human males are completely infertile, and 25-30% of those men are infertile because of causes on the Y chromosome--usually rearrangements or deletions. "We cannot overlook this major factor as non-existent in horses," said Chowdhary, "I suspect something similar is happening in them, too." After looking into available research, Chowdhary saw that very little is known about the genetic causes associated with reduced fertility or infertility in stallions. While he's examining these causes on other chromosomes, he's specifically targeting the horse Y chromosome and the genetic factors it harbors for stallion fertility.

"You cannot compare the Y chromosome to any of the autosomes (chromosomes not directly involved in sex determination)—the Y chromosome is an unusual, bizarre, amazing assembly of genetic material," he said, explaining that it is well-studied in the human and mouse, but that "the bottom line is our knowledge of the equine Y chromosome is negligible."

The long-term aim of sequencing the equine Y chromosome could lead to understanding the structure, organization, and function of Y-specific genes, identifying their role in stallion infertility, and developing a diagnostic test to accurately identify foals with potential Y-associated fertility problems. However, the project requires dedicated funding to rapidly understand the dynamics."A lot of economy is generated by the foals produced," said Chowdhary. "Male fertility is of prime significance, because stud fees can range from thousands to half a million."

Currently, he and other researchers are trying to get all of the tools and resources in place in the horse map of the Y chromosome. Interestingly, much can be drawn from the human gene map of the Y chromosome, helping the researchers identify and understand equine genes on that chromosome. "The bottom line is: first of all you need to build a (Y chromosome) map--this is essential. It is like a foundation without which you cannot build a house". He gratefully acknowledged the critical funding provided by American Quarter Horse Foundation to facilitate this initial stage of the work.

"Genomics is not a magic pill that can create a dream horse -- that is a misconception," he added. "My research is not trying to make unusual splashes of coat colors on horses, make mares have four foals, help develop a flu-like shot that will cure all diseases in a split second, nor am I making horses disease-free and resistant to all pathogens within a day. I would like people to accept genomics simply as an additional powerful future tool in the pocket of veterinarians that will make diagnoses more accurate, more reliable, and once done, lead to new ways to treat horses better."

"Identification of genetic background for various diseases and conditions in animals will shape veterinary practice (diagnosis as well as treatment) in the 21st century, just like antibiotics and vaccines did in the 20th century," said Chowdhary. "By funding critically important basic research in horse genomics, the equine community can make a huge difference in facilitating a leap into the future. They are the end-users and they will reap the most benefits of genetics research."

Editor's note: In the April issue, we called for help in gathering blood samples for Chowdhary's research. He said he had some response and has already received eight samples, but could use more. Click here for the type of horses to be sampled, and contact information.

About the Author

Stephanie L. Church, Editor-in-Chief, received a B.A. in Journalism and Equestrian Studies from Averett College in Danville, Virginia. A Pony Club and 4-H graduate, her background is in eventing, and she is schooling her recently retired Thoroughbred racehorse, Happy, toward a career in that discipline. She also enjoys traveling, photography, cycling, and cooking in her free time.

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